Not sure if this is related since this was a while back and it was on version 8.5 that I noticed this. If I recall there is was an issue with the SME cluster talking to the leaf cluster. If the SIP Trunk was talking to a specific node in the cluster, the intra cluster signaling had some problems. i.e. if you had Sub-A and Sub-B and if the SME Cluster sent the invite to Sub-A while the phone sat on Sub-B, the call wouldn't go through. It had to do with making sure that the Call Manager Group assigned to the device pool that was assigned to trunk had all CCM nodes in the cluster added to it. This became an irritation when I was working on it because we had more than 3 CCM Subscribers and there is a limit of 3 CCM nodes per Call Manager group and we had 4 CCM subs in the cluster which means we would never be able to send calls to a phone in the leaf cluster registered to the 4th node that wasn't part of the CCM Group for the device pool used by the SIP trunk between the Leaf and SME cluster.
You are dealing with 10.5 and you also mention that this was working prior, so I am not sure if this is the same, but I would certainly play around with the CMG for the device pool of the trunk and the CM that the phone registers to.
